Method and device for retrieving information from an optical record carrier at various reading speeds

ABSTRACT

The present invention is related to a method and a reading device ( 1 ) for retrieving information from an optical record carrier ( 10 ) in which the read power level of a radiation beam ( 3 ) for scanning the optical record carrier is set in dependence on the read-out speed. The invention is also related to a record carrier for use with such a method and a reading device.

FIELD OF THE INVENTION

This invention relates to a method of retrieving information from anoptical record carrier comprising the steps of setting a power level ofa radiation beam to a predetermined read power level, scanning therecord carrier by the radiation beam, detecting a reflected radiationbeam reflected from the record carrier, and retrieving the informationfrom the reflected radiation beam.

This invention further relates to a reading device for carrying out sucha method comprising a radiation source for generating a radiation beam,a setting means for setting a power level of the radiation beam to apredetermined read power level, a scanning means for scanning the recordcarrier by the radiation beam, a detector for transferring a reflectedradiation beam, said reflected radiation beam reflected from the recordcarrier, into an electrical signal, and a decoding means for retrievingthe information from said electrical signal.

BACKGROUND OF THE INVENTION

In optical record carriers information is encoded in a pattern ofoptically detectable marks and of spaces between the marks in aninformation layer of the optical record carrier. These marks may be inthe form of embossed pits, such as for example in Read Only type recordcarriers, in the form of changed optical properties of a dye layer, suchas for example in Recordable type record carriers, or in the form ofamorphous areas in a crystalline layer, such as for example inRewritable type media. In general these marks are stored along a spiralshaped or concentric shaped track on the information layer of a disclike optical record carrier. An optical record carrier may comprise asingle information layer or multiple information layers separated byspacer layers, such as for example a Dual Layer disc consisting of twoinformation layers.

In a reading device information is retrieved from such an optical recordcarrier by irradiating the information layer of the optical recordcarrier by a radiation beam, such as for example a laser light beam, anddetecting the radiation beam reflected from the optical record carrier.When the radiation beam scans the information layer along the track, thereflected radiation beam is modulated according to the pattern of marksand spaces stored on the information layer. This modulated reflectedradiation beam is transferred into a modulated electrical signal by adetector. By decoding this modulated electrical signal the informationstored on the optical record carrier is retrieved.

The Signal-to-Noise Ratio (SNR) of the modulated electrical signalrepresenting the information read from the optical record carrierdepends on the optics and electronics of the reading device. In generalthe SNR of the modulated electrical signal decreases with increasingreading speed. This because at increasing reading speeds the bandwidthof the detection electronics in the reading device needs to increase,thereby increasing all kinds of noise contributions, such as noise ofthe radiation source, electronic noise, shot noise, etcetera.

When reading optical record carriers, a more optimum SNR is obtained incase of high radiation levels. The signal level of the modulatedreflected radiation beam, and thereby the signal level of the modulatedelectrical signal, is proportional toP_(r)·M·R,where P_(r) is the power of the radiation beam (often referred to as theread power), M is the modulation of the marks and spaces, and R is thereflectivity of the information layer.

It is a problem of current optical systems, especially when usingoptical record carriers having a low reflectivity, such as for exampleDual Layer DVD Rewritable systems and Recordable and Rewritable BluRayDisc systems, that the Signal-to-Noise Ratio of the modulated electricalsignal representing the information read from the optical recordcarrier, often referred to as the read-out signal, becomes too low athigher read-out speeds when using conventional read powers, thus causingerrors in the retrieved information. This is especially the case in lowcost systems, where, in general, the efficiency of the optical path fromthe record carrier to the detector is low.

In future optical systems where the number of information layers in theoptical record carrier is increased to three or more, theSignal-to-Noise Ratio of the read-out signal may even further decreasedue to the decreasing reflectivity of the individual information layerswith increasing number of stacked information layers in a recordcarrier.

OBJECT OF THE INVENTION

It is an object of the present invention to provide a method and areading device that provide a modulated electrical signal having asufficiently high Signal-to-Noise Ratio at higher read-out speeds,thereby allowing the information to be reliably retrieved even at thesehigh read-out speeds.

SUMMARY OF THE INVENTION

This object is achieved by providing a method according to the openingparagraph which is characterized in that the power level of theradiation beam is set in dependence on a read-out speed to a read powerlevel of((1−α)+α√{square root over (n)})·P_(N)where α is a predetermined parameter with 0.2<α<0.8, P_(N) is a readpower level at a nominal read-out speed, often referred to as referenceread-out speed, and n is the actual read-out speed divided by thenominal read-out speed. P_(N) is usually expressed in mW. This object isfurther achieved by providing a reading device according to the openingparagraph which is characterized in that the setting means is arrangedfor setting the power level of the radiation beam in dependence on aread-out speed to a read power level of((1−α)+α√{square root over (n)})·P_(N)where α is a predetermined parameter with 0.2<α<0.8, P_(N) is a readpower level at a nominal read-out speed, and n is the actual read-outspeed divided by the nominal read-out speed.

According to the present invention it is proposed to increase the readpower level of the radiation beam as a function of the read-out speed,thereby compensating for the negative influence of the bandwidth of theelectronics and/or the reduced reflectivity of the information layer onthe Signal-to-Noise Ratio. Normally increasing the read power level ofthe radiation beam would at a certain power level result in thedegradation of the read stability, i.e. the number of times a recordcarrier can be read repeatedly without a signification deterioration ofthe record carrier. Therefore a maximum read power level is specifiedfor the various optical systems. However, the inventors had the insightthat this maximum read power level increases with an increasing read-outspeed, thereby allowing higher read power levels to be used at higherread-out speeds without degrading the read stability.

It is noted that from a theoretical point of view a read power levelwhich increases proportional to √{square root over (n)} would beindicated. However the inventors had the further insight that such adependency between read power level and read-out speed could result indestructive read power levels at high read-out speeds, resulting in apermanent loss of information on the record carrier being read.According to the present invention the read power level increases lessthan proportional to √{square root over (n)}, i.e. α has a value lessthan one. This allows for sufficiently high read power levels at allread-out speeds, without destroying information on the record carrier.

It is noted that the object of the present invention is achieved withoutthe need for expensive and/or complex electronic or optical components.

The values of the parameters α and P_(N) generally depend on propertiesof the record carrier. It is noted that when reading the differentinformation layers of a multi-layer record carrier, each individualinformation layer may have its own parameters values associated to it.It is further noted that the nominal read-out speed, at which the readpower level P_(N) is specified, is generally chosen to be the,so-called, 1× read-out speed corresponding to n=1. This 1× speed is thelowest read-out speed specified in the optical system.

The manufacturer of the optical disc may determine these parametervalues during the manufacturing process. Subsequently these values haveto be communicated to a reading device. This may be achieved byproviding these values in an area for holding record carrier relatedinformation on the record carrier itself, e.g. in the so-called ADIP,which area can be read out by the reading device. Alternatively, thevalues of the parameters α and P_(N) are specified in a specification ofthe optical system. Now, the manufacturer of the optical disc shouldadhere to these specifications during the manufacturing process of therecord carrier.

In preferred embodiments of the invention the value of α is in the rangebetween 0.33 and 0.5.

In embodiments of the invention for use with Dual Layer DVD Rewritableoptical record carriers (DVD+RW DL) especially good values for theparameters α and P_(N) are α=0.462 and P_(N)=1.3 mW, resulting in a readpower level of 0.7+0.6√{square root over (n)} mW, or, alternatively,α=0.385, resulting in a read power level of 0.8+0.5 √{square root over(n)} mW.

It should be noted that a read power level of((1−α)+α√{square root over (n)})·P_(N)in fact is an optimum value of the read power level, being an optimumtrade off between increasing the Signal-to-Noise Ratio by increasing theread power level and maintaining read stability by preventingdestructively high read power levels. Therefore, the read power level of((1−α)+α√{square root over (n)})·P_(N)could also be considered the read-out speed dependent upper boundary ofa save operating area of read power levels; the fixed valued lowerboundary being at the read power level for the lowest read-out speed.Now any read power level below the upper boundary and above the lowerboundary can be selected from this save operating area, resulting in animproved, but not necessary optimum, read performance.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other objects of the invention will be apparent from andelucidated further with reference to the embodiments described by way ofexample in the following description and with reference to theaccompanying drawings, in which

FIG. 1 shows an embodiment of a reading device according to theinvention,

FIG. 2 are graphs showing the dependency of the read power level of theradiation beam on the read-out speed, and

FIG. 3 are graphs of experimental data, with fitted curves according tothe formula of the invention.

DETAILED DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a reading device 1 according to an embodiment of theinvention for retrieving information from an optical record carrier 10.In this example the optical record carrier 10 is shown to be aDual-Layer record carrier comprising two information layers (L0, L1). Itis however noted that the invention works equally well with single-layerinformation carriers and with multi-layer information carrierscomprising more than two information layers.

The-reading device 1 comprises a radiation source 2, such as for examplea diode laser, for generating a radiation beam 3, such as for example alaser light beam. This radiation beam 3 scans the optical record carrier10 by way of well know scanning means. These scanning means include, forexample, a motor (not shown) for rotating the record carrier about itscenter and a lens system 5, comprising a beam splitter 51 and anobjective lens 52, for forming the radiation beam into a focusing spot15 on an information layer of the record carrier. In FIG. 1 theradiation beam 3 is shown focused on a first information layer of therecord carrier located closest to the entrance surface of the radiationbeam, often referred to as the L0 layer. However, under control offocusing and tracking means 7, which are also considered to be part ofthe scanning means, the radiation beam may also be focused on the otherinformation layer, often referred to as the L1 layer.

When the record carrier is rotated, the focusing and tracking means 7ensure that the focal spot 15 scans a track on the information layeralong which the information to be retrieved is stored. A reflectedradiation beam 9 reflected by the information layer L0 is guided by thelens system 5 towards a detector 4. This reflected radiation beam 9 ismodulated according to the pattern of marks and spaces between the marksalong the track being scanned. The detector 4 converts the incidentreflected radiation beam 9 into one or more electrical signals. At leastone of these electrical signals, i.e. the modulated electrical signal13, has a modulation that is related to the information being read. Thismodulated electrical signal 13 is applied to decoding means 8 forretrieving the actual information itself.

The reading device 1 further comprises setting means 6 connected to theradiation source 2 for controlling the power level of the radiation beam3 generated by the radiation source. In prior art reading devices thesesetting means 6 set the power level of the radiation beam to apredetermined read power level when retrieving information from anoptical record carrier 10.

It is to be noted that when the reading device is combined with arecording device for writing information into a recordable opticalrecord carrier, the radiation source 2 may also be used for generating aradiation beam for writing the information into the recordable recordcarrier. In such a combined device the setting means 6 are generallyalso used for setting the power level of the radiation beam to a,generally higher, write power level or to a pattern of write powerlevels.

In a reading device according to the present invention the setting means6 are arranged to set the read power level of the radiation beam independence on the read-out speed, also referred to as the linearvelocity. It is to be noted that the setting means 6 may be implementedby several distinct hardware elements, and/or by means of a suitablyprogrammed controller.

In general, a faster read-out speed allows for a faster retrieval of theinformation from the optical record carrier. In Constant AngularVelocity (CAV) devices the angular velocity of disc-shaped recordcarriers remains constant while the read-out speed (that is, the linearvelocity) varies from the inner diameter of the disc to the outerdiameter of the disc.

According to the present invention the setting means 6 set the powerlevel of the radiation beam 3 to a read power level of((1−α)+α√{square root over (n)})·P_(N)where α is a predetermined parameter with 0.2≦α≦0.8, P_(N) is a readpower level at a nominal read-out speed, and n is the actual read-outspeed divided by the nominal read-out speed. The nominal read-out speedis generally chosen to be the so-called 1×, or reference, speed,corresponding to n=1.

In embodiments of the invention for use with Dual Layer DVD Rewritableoptical record carriers (DVD+RW DL) especially good values for theparameters α and P_(N), are α=0.462 and P_(N)=1.3 mW, or, alternatively,α=0.385 and P_(N)=1.3 mW. This is shown in the graphs of FIG. 2 wherethe vertical axis represents the read power level (in mW) and thehorizontal axis represents n, being the actual read-out speed divided bythe nominal read-out speed. The dashed curve 21 represents α=0.462resulting in a read power level curve of 0.7+0.6 √{square root over (n)}mW, while the drawn curve 22 represents α=0.385 resulting in a readpower level curve of 0.8+0.5 √{square root over (n)}mW.

The dashed line 24 represents a fixed read power level of 1.2 mW, whichis defined in the prior art as a nominal read power level for testingpurposes. This dashed line 24 can be considered the lower boundary of asave operating area of read power levels. The upper boundary of thissave operating area of read power levels being the drawn curve 22, or,alternatively, the dashed curve 21. Any read power level may now beselected from this save operating area, resulting, on the on hand, in animproved read performance and, on the other hand, in a maintained readstability.

In an alternative embodiment of the invention the manufacturer of theoptical disc 10 determines the parameter values during the manufacturingprocess and provides these values in an area for holding record carrierrelated information on the record carrier 10 itself. Such an area forholding record carrier related information is, for example, the knownADIP and ATIP. The reading device 1 shown in FIG. 1 now reads, forexample at a read-out speed of 1×, i.e. at the reference speed, and/orusing the defined nominal read power for testing purposes, the parametervalues from the record carrier itself communicates them, through signal11, to the setting means 6. The setting means 6 subsequently uses theseread parameter values for setting the read power level at the higherread-out speeds.

FIG. 3 shows graphs of experimental data, with fitted curves accordingto the formula of the invention. All measurement data can well be fittedusing this formula, showing that this a good expression to describe themaximum read powers.

A general requirement for repeated read is that a disc can be read out amillion times, while remaining within specification, e.g. of mark jitterlevel. In experiments on a DVD rewritable disc the number of read cyclesis determined for different read powers and read speeds. Usually readpower at which the disc can be read out a million times is determined byextrapolation of experimental data points obtained at higher read powerscorresponding to lower number of read cycles. The measurements in FIG. 3give the read power at which precisely 1 million read cycles can beachieved. In practical applications a lower maximum read power is chosento obtain margins for:

-   -   Accuracy for setting the read power    -   Higher disc temperatures    -   Lower modulation frequency of the laser (the lower the        modulation frequency, the higher the peak powers are)    -   Drive to drive differences (spot size, wavelength, etc.)

The two extreme situations in practice to obtain a margin are: using amultiplication factor or obtaining the margin by using an offset.Situations in between can occur as well. When the additional margin as afunction of speed n is given by a factor, in practice the value of Pnwill be chosen lower. When the additional margin as a function of speedis given by an offset in mW, in practice Pn will be chosen lower and αwill be chosen higher.

The invention claimed is:
 1. A method of retrieving information from anoptical record carrier comprising the steps of: setting a power level ofa radiation beam to a predetermined read power level, scanning therecord carrier by the radiation beam, detecting a reflected radiationbeam reflected from the record carrier, and retrieving the informationfrom the reflected radiation beam, wherein the power level of theradiation beam is set, in dependence on a ratio of actual read-out speeddivided by a nominal read-out speed, to a read power level of((1−α)+α√{square root over (n)})·P_(N) where α is a predeterminedparameter with 0.2 <α<0.8, and P_(N) is a read power level at a nominalread-out speed, and n is the actual read-out speed divided by thenominal read-out speed.
 2. A method as claimed in claim 1, wherein thevalue of α is in a range between 0.33 and 0.50.
 3. A method as claimedin claim 1, wherein said method further comprises a step of retrieving avalue of the parameter α from the optical record carrier, and whereinthe power level of the radiation beam is set in dependence on theretrieved value of the parameter α.
 4. A method as claimed in claim 1for retrieving information from a multi-layer optical record carrier,wherein the power level of the radiation beam is set for eachinformation layer individually, each information layer having apredetermined parameter α to it.
 5. A reading device for retrievinginformation from an optical record carrier comprising a radiation sourcefor generating a radiation beam, a setting means for setting a powerlevel of the radiation beam to a predetermined read power level, ascanning means for scanning the record carrier by the radiation beam, adetector for transferring a reflected radiation beam, said reflectedradiation beam reflected from the record carrier, into an electricalsignal, and a decoding means for retrieving the information from saidelectrical signal, the setting means is arranged for setting the powerlevel of the radiation beam in dependence on a ratio of actual read-outspeed divided by a nominal read-out speed to a read power level of((1−α)+α√{square root over (n)})·P_(N) where α is a predeterminedparameter with 0.2<α<0.8, and P_(N) is a read power level at a nominalread-out speed, and n is the actual read-out speed divided by thenominal read-out speed.
 6. A reading device as claimed in claim 5,wherein the value of α is in a range between 0.33 and 0.50.
 7. A readingdevice as claimed in claim 5, wherein said reading device furthercomprises retrieving means for retrieving a value of α from the opticalrecord carrier, and wherein the setting means are arranged for settingthe read power level in dependence on the retrieved value of theparameter α.
 8. Optical record carrier -comprising an area consisting ofpatterns of optically detectable marks and of spaces between the marksrepresenting parameter values indicative of properties of the recordcarrier, wherein said area comprises patterns of optically detectablemarks and of spaces between the marks representing a value of α for usein a method as claimed in claim
 3. 9. Optical record carrier as claimedin 8 comprising multiple information layers, wherein said area comprisespatterns of optically detectable marks and of spaces between the marksrepresenting values of the parameter α for each of the informationlayers.